US20070099149 entitled “Endodontic device and method of utilizing and manufacturing same” corresponding to IL 171705 in the name of the present applicant discloses an endodontic device for cleaning, filing or reaming root canals. The device includes one or more metallic, flexible strands having an edge, a working section, a connecting section and a coupling head connected thereto. The wire strands are coated along the working section with a thin layer of a binder having abrasive particles embedded therein. In the file disclosed by US20070099149 the flexible, longitudinal strands are so constructed that their outer surfaces serve as active filing surfaces.
The invention described in this publication heralded a series of improvements, some of which are described in WO 2008/102352 entitled “Endodontic File” and WO 2011/104705 entitled “Rotary Endodontic File with Frictional Grip”.
Common to both these publications is an endodontic file for use with a dental instrument having at least one cord and a helical wire wound around at least a major portion of the cord and having an abrasive outer surface. In WO 2011/104705 the handle is adapted to be gripped in a handpiece by friction alone. This act as a safety clutch that helps to prevent fracture of the endodontic file should it snag inside the root canal, since once the force required to rotate the drill exceeds the friction of the clutch, the tool can no longer rotate.
In some prior art endodontic files, it is known to make the central cord so rigid that the inner cord can unintentionally penetrate the side wall of the root canal during the root canal procedure.
In WO 2011/104705 the handle is formed separate from the file portion with a hollow bore as best shown in FIG. 2 thereof and the upper end of the file portion is bonded within the hollow bore. As well shown in FIG. 7 thereof, the central cord of the file portion is of slightly smaller diameter than the internal diameter of the helical coil, so that it is free to slide within the helical coil. This requires that the upper end of helical coil be fixedly attached to the end of the cord, for example by tightly coiling a few turns of the helical wire around the end of the cord as shown in FIGS. 2 and 3 prior to attachment of the handle. Furthermore, as best shown in FIGS. 2 and 3, the central cord protrudes forward of the helical wire toward the working or tip end of the endodontic file by a length of 3 to 6 mm. The central cord at the tip end of the file may be treated to form a globule such that the tip of the file serves as a guide, rather like a mole's snout, for guiding the file through a pre-formed cavity as shown in FIG. 5. However, the small diameter of the cord, typically 0.2-0.3 mm, and its relative rigidity, render the file subject to penetration through the wall of the dental cavity.
Conventional approaches to forming an abrasive surface are as described in WO 2008/102352 where an abrasive layer is composed of a binder containing abrasive particles and is then coated on the outer surface of the file. It has been found during use of such files that the small abrasive particles can separate from the binder and become dislodged in the tooth canal, which is obviously undesirable.